Summary of Work: The human genome is subject to spontaneous changes and environmental threats. While double strand breaks (DSBs) are a likely source of genome instability, their appearance and repair in human cells are poorly understood. Given the similarity of repair systems, their relevance to the human condition and the likely commonality of DNA At-Risk Motifs (ARMs), we are investigating mechanisms of ARMs- iduced DSBs in the yeast Saccharomyces cerevisiae. Our focus is on repeats, size, organization and divergence of repeats, and genetic controls of ARMs associated changes. The most common repeats in the human genome are Alus, comprising ~ 10% of total DNA. We investigated if human Alus can cause instability in yeast. Closely spaced inverted repeat (IR) Alus stimulated recombination nearly 1000-fold, while direct Alus had no effect. The recombination was strongly dependent on distance between the 332 bp Alus. The maximal level of instability caused by Alu repeats was observed for inverted repeats separated by a distance less than 20 bp. Diverged Alus, also stimulated recombination, with 95% and 85% identical IR Alus stimulating recombination 100 fold and 8-fold, respectively, in a mismatch repair independent fashion. The inverted Alus resulted in DSBs, possibly during replication or by enzymes that recognize novel strucures. We investigated the role of the Mre11/Rad50/Xrs2 complex involved in repair, homologous recombination and end joining in yeast; a similar complex exists in humans. DSBs could be detected in Dmre11 and in Drad50 mutants suggesting that the Mre11/Rad50/Xrs2 is involved in processing, not in DSB induction. Screening for mutations that specifically affect IR stability revealed a mms19 mutation that overcomes the barriers associated with divergence and distance between the Alus. The yeast results led to an analysis of Alus in humans. We found that that closely spaced, highly homologous inverted Alus are rare. The potentially unstable inverted Alu pairs that we identified can be tested directly in human cell lines for ability to stimulate instability. - DNA Damage, Chrosomes, DNA Repair, Meiosis, Models Genetic, Saccharomyces, E. Coli